It has become increasingly common to apply a curable coating, such as a polyurethane, to an exposed surface such as a wall, floor or automobile body to offer protection against, for example, corrosion, moisture or abrasion. These coatings are often applied by spraying, rolling or painting the coating material on to the surface to be protected, and allowing the coating material to dry or cure in place.
Some polyurethane coatings as well as other high strength coatings are available for application in the form of a single component formulation.
Many commercially useful coating materials, such as paints, epoxies, varnishes, polyurethanes and other coating materials are available in the form of, and are formed from, two or more components which may be blended together immediately before application and applied to the surface to be coated by a dynamic mix spray gun. The components may be separately fed to the spray gun and mixed in the gun just before the coating material is sprayed on the surface to be coated. This procedure, described in more detail in, for example, the applicant's U.S. Pat. No. 5,388,761, provides a composition which will react on mixing to form a generally stable, substantially solid material soon after application to the surface to be coated, thus minimizing drying and curing time, and permitting the application of the coating material to vertical and other non-horizontal surfaces. A properly trained operator can apply a coating of relatively uniform thickness to almost any appropriate surface.
In the case of some of these materials, such as polyurethane, the liquid components may be selected to react with one another almost immediately to create an essentially solid, form-retaining product soon after contact with the surface to be coated. Therefore, the components are most commonly kept separated from one another and mixed together in the spray gun immediately before a coating of the material is to be applied to the surface.
The ratio of various components can be varied to provide the desired curing time and rate. For example, in the case of polyurethane, the two relevant components--isocyanate and polyol--may be prepared in a variety of formulations depending upon the application. Such formulations are often intended to be combined in the 1:1 ratio by volume. However, other mixing ratios, such as 5:1 and 1:5, are not uncommon. The appropriate mixing ratio for any particular application may also vary with environmental conditions, such as temperature, which affects the reactivity of the materials, viscosity or other physical or chemical properties of the components of the mixture.
Applying such a rapidly drying or curing mixture to a surface to be protected permits a quick and relatively uniform application of the coating material to the entire surface and shortens the time required before the coated surface may be put to its normal or intended use. However, the coating must be applied relatively quickly, and applying the mixture by spraying, rolling or painting often requires masking those areas of the surface that are not intended to be coated before application of the coating, to protect those areas from unwanted coating material. Subsequent trimming of the coating material is common to remove unwanted coating material after the coating is applied, either to provide access to the areas that ought not to be coated, such as drains or electrical outlets, or to provide a neat appearance.
One particularly useful application is the increasingly common use of spray-on coatings for liners of boxes of pick-up trucks, and interiors of vans and trucks. This application is one in which the appearance of both the coated and uncoated surfaces is particularly important, and one in which a significant amount of masking may be required. Such a spray-on liner provides protection against the corrosive elements in the atmosphere and also against the abrasion caused by various materials that may be carried in the truck, van or box.
These spray on linings have several advantages over the more conventional protection afforded by premoulded plastic liners that are inserted into the box of a pickup truck. Premoulded plastic liners do not form a water-tight seal with the body of the truck, and permit the entry of water and dirt between the liner and the truck body. This may result in substantial abrasion and corrosion to the body of the truck which is, however, not visible through the opaque liner. The loose fit of the liner results in movement of the liner against the body of the truck, increasing the abrasion damage to the truck body.
Spray-on linings, however, provide a coating, typically of polyurethane, that is tightly bonded to the truck body, and which does not permit the entry of dirt or moisture between the lining and the truck body. Also, the flexible properties of the polyurethane coating offer a slip resistant as well as protective surface for the cargo to ride on. In the case of a lining for a pick up truck box, the lining is generally applied to the floor and side walls of the box and to some portion of the top rails and side body. It is important to provide a neat edge along the perimeter of the box. The rear of the box is generally masked to avoid applying any coating to the hinges and latching mechanism, and the tail gate is generally removed and the surface facing into the box of the pickup truck is coated separately. Both this surface, and the ends of the side and bottom surfaces of the box must be trimmed to permit proper opening and closing of the door as well as providing a neat appearance.
As in the case of painting or other surface applications, the surface area that is actually covered by the sprayed on material may be determined by masking the surface that is not intended to be covered with masking tape and other commonly used masking material. The material to be sprayed on the surface is intended to adhere firmly to the surface. The use of masking materials prevents contact between those portions of the surface that are not intended to be covered, and allows the rapid application of the material only to the surface which is intended to be covered. In these operations, masking tape or other masking material is used, which has an adhesive coating that is sufficiently strong to hold the masking material in place while it is intended to be there, and yet permits the easy removal of the masking material when it is no longer required, while leaving no significant amount of adhesive material on the surface to be protected. The use of the term adhesive throughout this application generally refers to a removable adhesive having these general properties.
After the application of the coating material, however, some trimming is required to remove the coating material. This is commonly done by cutting the coating material along the boundary of the masked area, to separate the coating that is to remain in place, and which will be firmly bonded to the truck body, from the coating material that is to be removed, which should not have contacted the truck body and which should be separated from the truck body by the masking material. Once this separation is made, it is possible to remove the masking tape or other masking material, and the unwanted surface coating. It is thus important in such a trimming application to cut precisely along the edge of the masking material so that no masking material is left on the surface beneath the coating. This would result in a portion or area of coating material that is not adhered to the surface to be protected, which could subsequently result in the peeling of the protective coating from the surface. Conversely, if the cut is away from the masked edge and into the area which is intended to be coated, removal of the coating from the masked area will be more difficult and may result in the removal of paint from the truck body.
One difficulty posed by the use of the relatively thick, abrasion-resistant coatings, such as polyurethane coatings, is the difficulty in locating the edges to be trimmed. Furthermore, while the removal of masking material used in painting effectively acts as an edge trimming method, tearing or cutting the paint layer as the masking material is removed, conventional masking materials will not tear through the polyurethane coating, and often cannot be located under the relatively thicker coatings of polyurethane such as those used to line a truck box.
Various methods have been developed to overcome this difficulty. For example, several layers of masking tape may be used and layers removed sequentially so that each layer of masking tape removes individual thin layers of the coating material before the coating begins to cure. This procedure generally requires extra personnel, is a time-consuming method that leaves a relatively rough edge to the coating material as well as an inferior bond at the extreme edge of the coating.
The most common way of trimming such coatings is simply by cutting the coating along the edge of the masking material with a knife or other sharp instrument. This requires, first of all, locating the edge of the masking material, and then cutting the protective coating with a sharp instrument such as a knife. This almost invariably has the result of cutting or scoring the underlying surface, which is a particular problem with painted surfaces such pick-up truck beds and requires that the line cut or scored into the truck bed to be repainted before the vehicle can be delivered to the consumer.